Effect of ion-crosslinking on supercritical CO2 foaming behavior and foam properties of EVA/ZnO composites

How to realize the recycling of materials is an important topic for polymer materials, especially crosslinked polymers. In this work, by means of the reaction of zinc oxide (ZnO) and maleic anhydride group, ethylene-vinyl acetate copolymer (EVA)/ZnO composites with reversible ion-crosslinking structure were prepared by melt mixing method. The gel content and the degree of ion crosslinking in EVA were tracked using the THF extraction method. The presence of ion-crosslinking structure strongly affected the foaming behavior of EVA composites in supercritical CO2 (sc-CO2) molding foaming process. The influence of the ion crosslinking on the structure and properties of EVA foams was studied by means of SEM and compression set test etc. The results showed that the appropriate degree of ion-crosslinking was beneficial to increase the volume expansion rate and cell density of the obtained foams, and reduce the average cell diameter and compression set of the foams. The obtained EVA composite foams showed good resilience and melt-reprocessing performance. However, too high ion-crosslinking degree severely limited the growth of cells due to the difficulty of molecular chain movement. At the same time, the excessive ZnO in the matrix had an adverse effect on the compression set of EVA foam.